1. Field of the Invention
The present invention relates to an apparatus for controlling a lock-up clutch used in a power transmitting system of a motor vehicle.
2. Discussion of the Related Art
In a motor vehicle having a fluid-filled power transmitting device equipped with a lock-up clutch such as a torque converter or fluid coupling incorporating such a lock-up clutch, there is proposed an apparatus for controlling the lock-up clutch, such that the lock-up clutch is fully engaged when a running condition of the vehicle falls in a predetermined fully engaging area, for the purpose of improving the fuel economy of the vehicle while minimizing the power loss. The running condition of the vehicle is represented in a two-dimensional coordinate system wherein a load of an engine of the vehicle is taken along one axis while a running speed of the vehicle is taken along another axis. The fully engaging area indicated above is suitably located within the two-dimensional coordinate system. An example of such a lock-up clutch control apparatus is disclosed in JP-A-62-274157 (published in 1987).
In the known lock-up clutch control apparatus of the type indicated above, the lock-up clutch may be switched from a fully engaged state into a fully released state when the vehicle running condition has changed to be outside the predetermined fully engaging area. That is, a pump impeller and a turbine impeller of the fluid-filled power transmitting device which have been directly connected to each other are rapidly disconnected or released from each other. Consequently, the torque transmitted from the engine to an automatic transmission through the lock-up clutch is temporarily rapidly reduced with a result of a change of the transmitted torque, irrespective of a manipulation of the vehicle by the operator of the vehicle. Such a torque change may cause a "releasing shock" of the vehicle, with which the vehicle operator is uncomfortable.